U.S. patent number 5,146,858 [Application Number 07/593,021] was granted by the patent office on 1992-09-15 for boiler furnace combustion system.
This patent grant is currently assigned to Mitsubishi Jukogyo Kabushiki Kaisha. Invention is credited to Shuzo Naito, Masaharu Oguri, Kimishiro Tokuda.
United States Patent |
5,146,858 |
Tokuda , et al. |
September 15, 1992 |
Boiler furnace combustion system
Abstract
A boiler furnace combustion system typically includes main
burners disposed on side walls of or at corners of a
square-barrel-shaped boiler furnace having a vertical axis, the
burner axes being directed tangentially to an imaginary cylindrical
surface coaxial to the furnace. Air nozzles are disposed in the
boiler furnace at a level above the main burners, so that unburnt
fuel left in a reducing atmosphere or a lower oxygen concentration
atmosphere of a main burner combustion region can be perfectly
burnt by additional air blown through the air nozzles. The present
invention provides two groups of air nozzles disposed at higher and
lower levels, respectively. The air nozzles at the lower level are
provided at the corners of the boiler furnace with their axes
directed tangentially to a second imaginary coaxial cylindrical
surface having a larger diameter than the first imaginary coaxial
cylindrical surface. And, the air nozzles at the higher level are
provided at the centers of the side wall surfaces of the boiler
furnace with their axes directed tangentially to a third imaginary
coaxial cylindrical surface having a smaller diameter than the
second imaginary coaxial cylindrical surface.
Inventors: |
Tokuda; Kimishiro (Nagasaki,
JP), Oguri; Masaharu (Nagasaki, JP), Naito;
Shuzo (Tokyo, JP) |
Assignee: |
Mitsubishi Jukogyo Kabushiki
Kaisha (Tokyo, JP)
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Family
ID: |
14673515 |
Appl.
No.: |
07/593,021 |
Filed: |
October 3, 1990 |
Foreign Application Priority Data
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Oct 3, 1989 [JP] |
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1-115882[U] |
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Current U.S.
Class: |
110/261; 110/263;
110/264; 110/347 |
Current CPC
Class: |
F23C
5/32 (20130101); F23C 7/02 (20130101); F23C
2201/101 (20130101) |
Current International
Class: |
F23C
5/32 (20060101); F23C 5/00 (20060101); F23C
7/02 (20060101); F23C 7/00 (20060101); F23K
005/00 (); F23D 001/00 () |
Field of
Search: |
;110/261,263,245,347 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2837156 |
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Mar 1979 |
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DE |
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8525256.5 |
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Feb 1987 |
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DE |
|
Primary Examiner: Yuen; Henry C.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. In a boiler having a vertically extending square barrel-shaped
furnace formed by side walls intersecting at corner portions and
defining a longitudinal axis centrally thereof, a combustion system
comprising:
a plurality of main burners disposed nearly horizontally on the
side walls or at the corner portions of the furnace, said main
burners defining axes along which fuel is injected into a main fuel
combustion region of the furnace by the main burners, said axes of
the main burners extending tangentially to an imaginary cylinder
coaxial with the furnace;
fuel supply means and air supply means for supplying fuel to said
main burners and introducing air into the main fuel combustion
region in amounts sufficient to produce a reducing atmosphere or an
atmosphere of a low oxygen concentration of 1% or less in the main
fuel combustion region;
at least one group of air nozzles located at a lower level above
the main fuel combustion region for injecting additional air into
the furnace above the main combustion region, and air supply means
for blowing air through said air nozzles disposed at the lower
level,
the air nozzles at said lower level being disposed at said corner
portions of the furnace and defining axes, respectively, along
which additional air is injected into the furnace,
the axes of said air nozzles at said lower level extending
tangentially to a second imaginary cylinder coaxial with the
furnace and having a diameter larger than that of said first
imaginary cylinder; and
at least one group of air nozzles located at an upper level above
said lower level for also injecting additional air into the
furnace, and air supply means for blowing air through said air
nozzles at the upper level,
the air nozzles at said upper level being disposed at portions of
the side walls of the furnace located centrally of the corner
portions, respectively, and defining respective axes along which
additional air is also injected into the furnace,
the axes of said air nozzles at said upper level extending
tangentially to a third imaginary cylinder coaxial with the furnace
and having a diameter smaller than that of said second imaginary
cylinder.
2. A combustion system in the furnace of a boiler as claimed in
claim 1, wherein said air supply means blows air through said air
nozzles at an additional air flow rate of between 10% to 40% of a
total flow rate of combustion air, wherein said total flow rate is
the sum of the flow rate at which air is introduced into the main
fuel combustion region and said additional air flow rate.
3. A combustion system in the furnace of a boiler as claimed in
claim 1, wherein a common source of air constitutes said air supply
means.
4. A combustion system in the furnace of a boiler as claimed in
claim 1, wherein separate sources of air constitute the air supply
means for supplying air to said air nozzles and the air supply
means for introducing air into said main fuel combustion region,
respectively.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a boiler furnace combustion
system, and more particularly to improvements in an electric
utility or industrial boiler furnace combustion system.
2. Description of the Prior Art:
At first, one example of a boiler furnace in the prior art will be
explained with reference to FIGS. 5 to 7. Among these figures, FIG.
5 is a vertical cross-sectional view; FIG. 6 is a horizontal
cross-sectional view taken along line VI--VI in FIG. 5; and FIG. 7
is another horizontal cross-sectional view taken along line
VII--VII in FIG. 5.
In these figures, reference numeral 01 designates a boiler furnace
main body, numeral 02 designates main burner wind boxes, numeral 03
designates main burner air nozzles, numeral 04 designates main
burner fuel injection nozzles, numeral 05 designates air ducts for
introducing air to the main burners, numeral 06 designates fuel
feed pipes, numeral 07 designates additional air ducts, numeral 09
designates flames, numeral 10 designates air for the main burners,
numeral 11 designates fuel such as pulverized coal, petroleum,
gaseous fuel or the like, numeral 12 designates additional air,
numeral 13 designates unburnt combustion gas, numeral 14 designates
combustion exhaust gas, numeral 15 designates wind boxes, numeral
16 designates air nozzles, and numeral 20 designates imaginary
cylindrical surfaces.
At lower corner portions of a square-barrel-shaped boiler furnace
main body 01 having a nearly vertical axis are respectively
provided main burner wind boxes 02, and at upper corner portions of
the same main body are respectively provided wind boxes 15 for
additional air (hereinafter abbreviated as AA). Within each main
burner wind box 02 there is provided main burner fuel injection
nozzles 04 and main burner air nozzles 03 extending nearly
horizontally.
Fuel 11 is fed from a fuel feed installation (not shown) to the
main burner fuel injection nozzles 04 through the fuel feed pipes
06 and is injected into the boiler furnace 01. On the other hand,
main burner air 10 is fed from a ventilating installation (not
shown) through the main burner air ducts 05 to the main burner wind
boxes 02, and is blown into the boiler furnace 01 through the main
burner air nozzles 03.
The injection of the fuel 11 and of the main burner air 10 is
effected in a direction tangential to an imaginary cylindrical
surface 20 which is located at the central portion of the boiler
furnace 01. The fuel 11 injected into the boiler furnace 01 along
the tangential direction is ignited by an ignition source (not
shown) to form flames 09, and as the fuel diffuses and mixes with
the main burner air 10 injected in the tangential direction through
the main burner air nozzles 03, combustion is continued.
The main burner air 10 is fed at a rate lower than an air feed rate
that is theoretically necessary for combusting the fuel 11 injected
into the boiler furnace 01. Therefore, the interior portion of the
boiler furnace 01 below the AA blowing portion is held under a
reducing atmosphere. Accordingly, the combustion of the fuel 11
produces unburnt combustion gas 13 containing unburnt fuel at the
portion below the AA blowing portion.
The AA 12 is fed from a ventilating installation (not shown) which
also feeds the main burner air 10, or from a separately disposed
ventilating installation (not shown) through the AA ducts 07. The
AA 12 is blown into the boiler furnace 01 in a tangential manner,
like the main burner air 10, through the AA air nozzles 16 disposed
nearly horizontally in AA wind boxes 15. Normally, the injection of
the AA 12 is effected in the same tangential direction as the main
burner air 10 with respect to the imaginary cylindrical surface 20.
The flow rate of the AA 12 is such that a sufficient amount of
oxygen, i.e. an amount necessary for perfectly burning unburnt fuel
in the unburnt combustion gas 13, is fed into the boiler furnace
01.
The AA 12 blown into the boiler furnace 01 is mixed with the
unburnt combustion gas 13 by diffusion, thus causing the unburnt
fuel in the unburnt combustion gas 13 to burn perfectly, and is
exhausted to the outside of the boiler furnace 01 as combustion
exhaust gas 14.
In such a boiler furnace in the prior art, the combustion of the
fuel 14 injected through the main burner fuel injection nozzles 04
produces some unburnt combustion gas 13 due to the fact that the
flow rate of the main burner air 10 is less than the theoretical
air flow rate. And, the interior portion of the boiler furnace
below the AA blowing portion is under a reducing atmosphere.
Consequently, in that portion below the AA blowing portion, the
amount of nitrogen oxides (hereinafter represented by NO.sub.x)
produced by the combustion of the fuel 11 is small, and instead
intermediate products such as ammonia (NH.sub.3), cianic acid (HCN)
and the like are produced.
Subsequently, in the AA blowing portion, it is desired to
completely combust unburnt components of the unburnt combustion gas
13 by injecting AA 12 through the AA blowing nozzles 16. At that
time since the intermediate products such as NH.sub.3, HCN and the
like tend to be oxidized and transformed into NO.sub.x, the
injection of AA 12 is carried out in a relatively low-temperature
(about 1000.degree.-1200.degree. C.) atmosphere within the boiler
furnace 01 for the purpose of suppressing the transformation rate
of the intermediate products into NO.sub.x.
And because the flow rate of the main burner air 10 is less than
the theoretical air flow rate necessary for the air to completely
combust with the fuel 11, the unburnt combustion gas 13 rises while
swirling. As the unburnt combustion gas 13 rises, the outer
diameter of the swirling flow of the unburnt combustion gas 13
gradually becomes large, and in the proximity of the AA blowing
portion, the amount of unburnt combustion gas 13 flowing along the
wall of the boiler furnace 01 increases.
The blowing momentum of the AA 12 is about 1/5 to 1/3 that of the
blowing momentum of the main burner air 10, provided that the
blowing velocities are equal to each other. The AA 12 blowing
through the AA blowing nozzles 16 at the respective corner portions
both diffuses and mixes with the main flow portion of the unburnt
combustion gas 13, and penetrates through the main flow portion and
flows towards the central portion of the boiler furnace 01. The
momentum of the AA 12 flowing towards the central portion of the
boiler furnace 01 is attenuated due to the facts that the AA 12 has
penetrated through the main flow portion of the unburnt combustion
gas and that the distance from the AA blowing nozzle 16 to the
central portion of the boiler furnace 01 is long. Hence, the AA 12
does not diffuse or mix with the unburnt combustion gas 13 in the
proximity of the central portion of the boiler furnace 01.
Accordingly, the AA 12 rises without contributing to the completion
of the combustion of the unburnt combustion gas, and it is
exhausted from the outlet of the boiler furnace 01.
Therefore, in order to complete the combustion of the unburnt
components of the unburnt combustion gas 13 within the boiler
furnace 01 in the prior art, countermeasures such as (1) increasing
a total combustion air flow rate (a flow rate of main burner air 10
+ a flow rate of AA 12), (2) lengthening the time in which it takes
combustion gas from the AA blowing portion to flow to the outlet of
the boiler furnace 01, (3) weakening the reducing atmosphere under
the AA blowing portion by increasing a flow rate of the main burner
air 10, or the like are necessary. However, countermeasures (1) and
(3) are disadvantageous in view of the production of NO.sub.x, and
the countermeasure (2) is disadvantageous in view of cost.
As described above, the boiler furnace combustion system in the
prior art presents problems in connection with the diffusion and
mixing of the AA 12 and the unburnt combustion gas 13. Therefore,
there is a problem to be resolved in that if one intends to
decrease NO.sub.x production, the amount of unburnt fuel is
increased, while if one intends to decrease the amount of unburnt
fuel remaining, NO.sub.x reduction is not sufficient.
SUMMARY OF THE INVENTION
It is therefore one object of the present invention to provide an
improved boiler furnace combustion system, in which both an unburnt
fuel component and an NO.sub.x content in combustion exhaust gas
are low and which does not require a large installation cost.
The boiler furnace combustion system includes a plurality of main
burners disposed nearly horizontally on side wall surfaces or at
corner portions of a square-barrel-shaped boiler furnace having a
vertical axis with axes of the burners directed tangentially to a
cylindrical surface having its axis aligned with the axis of said
boiler furnace, and a plurality of nozzles for injecting additional
air and disposed nearly horizontally in said boiler furnace at a
higher level than said main burners. A main burner combustion
region, in which fuel from said main burners and air are injected,
is held under a reducing atmosphere or an atmosphere of a low
oxygen concentration of 1% or less, and that fuel not burnt in said
main burner combustion region is perfectly burnt by the additional
air blown through said nozzles. The system is characterized in that
said plurality of nozzles for injecting additional air are provided
in at least two groups at upper and lower levels of the boiler
furnace, respectively. The nozzles for injecting additional air at
the lower level are provided at corner portions of said boiler
furnace and have their nozzle axes directed tangentially to a
second cylindrical surface having its axis aligned with the axis of
said boiler furnace and having a larger diameter than that of first
said cylindrical surface. The nozzles for injecting additional air
at the higher level are provided at central portions of the side
wall surfaces of said boiler furnace and have their nozzle axes
directed tangentially to a third cylindrical surface having its
axis aligned with the axis of said boiler furnace and having a
smaller diameter than that of said second cylindrical surface.
According to the present invention, since the temperature of the
unburnt combustion gas becomes lower as the gas nears a furnace
wall, by blowing additional air through the air nozzles (lower
level) provided at the corner portions of the boiler furnace in the
direction tangential to the second cylindrical surface close to the
wall surface and having a larger diameter, the additional air is
reliably diffused and mixed with the unburnt combustion gas. In
addition, by blowing additional air through the air nozzles (higher
level) provided at the central portions of the side wall surfaces
of the boiler furnace in a direction tangential to the third
cylindrical surface having a smaller diameter than that of the
second cylindrical surface, that is, towards the central portion of
the boiler furnace, the unburnt combustion gas and additional air
are diffused and mixed uniformly in a reliable manner.
The above-mentioned and other objects, features and advantages of
the present invention will become more apparent by referring to the
following description of preferred embodiments of the invention
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a longitudinal cross-sectional view of one preferred
embodiment of the present invention;
FIG. 2 is a transverse cross-sectional view of the same taken along
line II--II in FIG. 1;
FIG. 3 is another transverse cross-sectional view of the same taken
along line III--III in FIG. 1;
FIG. 4 is still another transverse cross-sectional view of the same
taken along line IV--IV in FIG. 1;
FIG. 5 is a longitudinal cross-sectional view of one example of a
boiler furnace in the prior art;
FIG. 6 is a transverse cross-sectional view of the same taken along
line VI--VI in FIG. 5;
FIG. 7 is another transverse cross-sectional view of the same taken
along line VII--VII in FIG. 5;
FIG. 8 is a diagram showing relationships between NO.sub.x
production rate and a soot/dust concentration versus an AA blowing
rate in both the illustrated embodiment and the prior art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
One preferred embodiment of the present invention is generally
shown in FIGS. 1 to 4. In these figures, reference numerals 01 to
14 designate component parts similar to those in the boiler furnace
in the prior art illustrated in FIGS. 5 to 7 and described
previously. On the other hand, reference numeral 115 designates
upstream side (lower level) AA wind boxes, numeral 116 designates
upstream side (lower level) AA nozzles, numeral 117 designates
downstream side (upper level) AA wind boxes, numeral 118 designates
downstream side (upper level) AA nozzles, numeral 119 designates
upstream side (lower level) AA (additional air), and numeral 120
designates downstream side (upper level) AA (additional air).
Fuel 11 sent from a fuel feed installation (not shown) through fuel
feed pipes 06 and main burner air 10 sent likewise from a
ventilating installation (not shown) through main burner air ducts
05, are respectively injected through main burner air fuel
injection nozzles 04 and burner air nozzles 03 into a boiler
furnace 01. The injection of the fuel 11 and of the main burner air
10 are effected in a tangential direction to an imaginary
cylindrical surface 20, having an axis aligned with the axis of the
boiler furnace 01 (see FIG. 2).
The fuel 11 injected into the boiler 01 is ignited by an ignition
source (not shown) and forms flames 09, and as it diffuses and
mixes with the main burner air 10 blown in the tangential direction
through the main burner air nozzles 03, combustion continues.
Here, the main burner air 10 is fed at a flow rate less than the
air flow rate that is theoretically necessary for combusting the
fuel 11 injected into the boiler furnace 01. Therefore, the
interior portion of the boiler furnace 01 below the AA blowing
portion is held under a reducing atmosphere. The combustion of the
fuel 11 produces unburnt combustion gas 13 containing unburnt fuel
due to a lack of oxygen in the interior portion below the AA
blowing portion, and the unburnt combustion gas rises while
swirling.
Above the main burner wind boxes 02 of the boiler furnace main body
01 is the AA blowing portion, divided into two groups respectively
disposed at higher and lower levels.
In the upstream side (lower level) AA blowing portion at which the
unburnt combustion gas 13 first arrives, the upstream side (lower
level) AA wind boxes 115 are provided at the respective corner
portions of the square-barrel-shaped boiler furnace main body 01.
Upstream side (lower level) A nozzles 116 extend nearly
horizontally within wind boxes 115 to inject the upstream side
(lower level) AA 119 into the flow of the unburnt combustion gas 13
which has risen. The injection of the upstream side (lower level)
AA 119 through the upstream side (lower level) AA nozzles 116 is
effected in a direction tangential to a second imaginary
cylindrical surface 21 having an axis aligned with the axis of the
boiler furnace 01 and having a larger diameter than the
above-mentioned imaginary cylindrical surface (see FIG. 3).
In the downstream side (upper level) AA blowing portion, the
downstream side (upper level) AA wind boxes 117 are provided at the
central portions of the respective side walls of the boiler furnace
main body 01. The downstream side (upper level) AA nozzles 118
extend nearly horizontally within wind boxes 117 to inject the
downstream side (upper level) AA 120 therefrom into the furnace 01.
The downstream side (upper level) AA 120 is injected in a direction
tangential to a third imaginary cylindrical surface 22 (see FIG. 4)
through the downstream side (upper level) AA nozzles 118. This
third imaginary cylindrical surface 22 has a smaller diameter than
the above-mentioned second imaginary cylindrical surface and its
axis aligned with the axis of the boiler furnace 01.
The flow rate of the AA 12 is 10% to 40% of a total combustion air
flow rate (a flow rate of main burner air 10 + a flow rate of AA
12). Because this air flow is separated into the upstream side AA
119 and the downstream side AA 120, blowing momenta of the upstream
side AA 119 and the downstream side AA 120 both become small
compared to that of the main burner air 10. With respect to the
upstream side (lower level) AA 119 blown from the respective corner
portions of the boiler furnace main body 01, since the distance
from the tip end of the blowing nozzle 116 to the central portion
of the boiler furnace 01 is long compared to the distance over
which the downstream side (higher level) AA 120 is blown from the
central portions of the respective side walls (about 1.4 times as
long as the latter in the case where the cross section of the
boiler furnace 01 is square), depending upon the blowing momentum
of the upstream side (lower level) AA 119, the blowing energy may
be attenuated and the AA may rise towards the outlet of the boiler
furnace 01 without forming a swirling flow and without being
sufficiently diffused and mixed with the unburnt combustion gas 13.
Accordingly, it is important that the upstream side (lower level)
AA 119 should be blown into the swirling flow of the unburnt
combustion gas 13 as early as possible immediately after it has
been blown into the furnace. This is one of the reasons why the
diameter of the second imaginary cylindrical surface 21 is set to
be larger than the diameter of the imaginary cylindrical surface
20.
The unburnt combustion gas rises while it is swirling, and as it
rises the outer diameter of its swirl flow becomes large.
Therefore, in the proximity of the upstream side (lower level) AA
blowing portion, a flow rate of the unburnt combustion gas 13
flowing along the walls of the boiler furnace 01 increases. Since
the unburnt temperature of the combustion gas 13 is lower as the
gas approaches the walls of the boiler furnace 01, in order to make
the unburnt component burn perfectly, it is necessary to quickly
feed oxygen to a region close to the walls of the boiler furnace
01. The upstream side (lower level) AA 119 is provided to surely
mix with the unburnt combustion gas 13 in order to perfectly burn
the unburnt component of this unburnt combustion gas 13 in the
proximity of the walls of the boiler furnace 01. And, this is also
the reason why the diameter of the second imaginary cylindrical
surface 21 is set to be larger than that of cylindrical surface
21.
In this way, the unburnt combustion gas 13 diffuses and mixes with
the upstream side (lower level) AA 119 in the proximity of the
walls of the boiler furnace 01, and while combustion continues, it
reaches the downstream side (higher level) AA blowing portion.
Since the downstream side (higher level) A 120 blows through the
downstream side (higher level) AA nozzles 118 provided nearly at
the central portions of the side walls of the boiler furnace 01,
the distance from the nozzles 118 to the third imaginary
cylindrical surface 22 at the central portion of the boiler furnace
01 is short. Hence, the blowing momentum attenuates only a little,
and therefore, the downstream side (higher level) AA forms a strong
swirling flow. Accordingly, the AA diffuses and mixes effectively
with the unburnt combustion gas 13 at the central portion of the
boiler furnace 01. Thus, an unburnt component of the unburnt
combustion gas 13 is burned perfectly, and is exhausted from the
outlet of the boiler furnace 01 as combustion exhaust gas 14.
As described above, in the illustrated embodiment, owing to the
facts that the AA blowing portion includes two groups of wind boxes
and nozzles disposed at higher and lower levels, respectively, and
that the upstream side (lower level) AA 119 is injected from the
respective corner portions of the boiler furnace 01 to the
proximity of the walls of the boiler furnace 01, while the
downstream side (higher level) AA 120 is blown from the central
portions of the respective side wall surfaces towards the central
portion of the boiler furnace 01, the AA 12 and the unburnt
combustion gas 13 can surely diffuse and mix with each other,
whereby a highly efficient combustion and reduction of the amount
of soot and dust can be realized. In addition, because a very
complete combustion can be expected to be effected by the AA 12,
the combustion under the AA blowing portion can be effected with a
lower air-to-fuel ratio than in the prior art.
FIG. 8 is a diagram showing relationships of an NO.sub.x production
rate and a soot/dust concentration versus an AA blowing rate with
respect to both the illustrated embodiment and the prior art. This
data is the result of tests conducted by the inventors on a test
furnace using pulverized coal as fuel. With respect to this data,
the relationship between the NO.sub.x production rate and the AA
blowing rate constitute generally well-known characteristics. In
the case where petroleum or gaseous fuel is used in place of the
pulverized coal, similar characteristics are also observed.
In FIG. 8, the left ordinate represents the proportion (%) of
NO.sub.x at the outlet of the furnace, and the right ordinate
represents a soot/dust concentration (mg/Nm.sup.3) in combustion
exhaust gas at the outlet of the furnace. Also, the abscissa
represents a ratio (%) of the AA flow rate to a total combustion
air flow rate.
As will be seen from FIG. 8, the amount of NO.sub.x at the outlet
of the furnace tends to become lower as the AA flow rate proportion
increases. However, in the boiler furnace combustion system in the
prior art, as the soot/dust concentration at the outlet of the
furnace reaches a soot/dust limit value (250 mg/Nm.sup.3) at an AA
flow rate proportion of 18%, the AA flow rate proportion cannot be
increased further. Therefore, the NO.sub.x production rate cannot
be suppressed to a lower value. In the illustrated embodiment,
however, the soot/dust concentration at the outlet of the furnace
reaches the soot/dust limit value when the AA blowing rate
proportion is 33%. Therefore, the NO.sub.x production rate is about
30% lower than that in the prior art.
This is due to the fact that as a result of employing a relatively
high AA flow rate proportion, that is, a low main burner air flow
rate proportion--a flow rate of main burner air 10/(a flow rate of
fuel 11 x a theoretical air flow rate)--a reducing atmosphere is
formed in the region below the AA blowing portion. Therefore, the
NO.sub.x produced by combustion of the fuel 11 is resolved and
transformed into nitrogen molecules N.sub.2 and intermediate
products such as NH.sub.3, HCN and the like. The proportion of
NO.sub.x being transformed into N.sub.2, NH.sub.3, HCN and the like
increases as an air-to-fuel ratio in the region below the AA
blowing portion decreases (however, at a ratio lower than a certain
air-to-fuel ratio, this phenomenon is reversed). While the NH and
HCN produced in the region below the AA blowing portion are
oxidized and retransformed into NO.sub.x by the AA 119 and 120, if
a reducing reaction in the region below the AA blowing portion is
effected efficiently and the AA 119 and 120 are flowing uniformly,
small proportions of NH.sub.3 and HCN are retransformed into
NO.sub.x, and the NO.sub.x production rate at the outlet of the
boiler furnace 01 is suppressed to a low value.
As described in detail above, in the illustrated embodiment, since
a highly efficient combustion can be carried out by the AA 190 and
120, the AA flow rate proportion can be set to a large value,
whereby a low NO.sub.x production rate, which could not be realized
in the prior art, can be achieved.
It is to be noted that while in the above-described embodiment the
AA is injected at two levels (upper and lower), in the case of a
large-capacity boiler in which the boiler furnace main body 01 is
large, the upstream side (lower level) AA nozzles 116 and the
downstream side (higher level) AA nozzles 118 could be provided in
a number of pairs.
According to the present invention, owing to the fact that the AA
is injected at least two upper and lower levels, and the upstream
side (lower level) AA is blown from the respective corner portions
of the boiler furnace into the unburnt combustion gas in the
proximity of the furnace wall surfaces, the unburnt combustion gas
and the AA are reliably diffused and mixed. In addition, taking
into consideration the fact that the temperature of the unburnt
combustion gas becomes lower as the gas nears the furnace wall
surfaces, the upstream side (lower level) AA is used to promote
combustion in the proximity of the wall surface, while the
downstream side (higher level) AA is used to promote combustion at
the central portion of the furnace. Therefore, a high combustion
efficiency is realized, and moreover, a low air-to-fuel ratio in
the main burner combustion zone (under the AA blowing portion) can
be maintained. As a result, low-NO.sub.x production and
low-unburnt-component combustion can be achieved.
While a principle of the present invention has been described above
in connection with one preferred embodiment of the invention, it is
intended that all matter contained in the above description and
illustrated in the accompanying drawings shall be interpreted to be
illustrative and not in a limiting sense.
* * * * *